Equalizers are widely used apparatus in communications systems to compensate for linear (amplitude and phase) distortion in the channel. Linear equalizers and decision feedback equalizers are two major equalizer classifications. Linear equalizers are more widely used than the decision feedback equalizers in many communications applications as they are simpler to implement and provide virtually the same compensation benefits. However, with the development of higher transmission speeds, i.e., above 19.2 kb/s for voiceband modems, a decision feedback equalizer provides a significant advantage over a linear equalizer and is definitely the preferred compensation apparatus because it is better suited to compensate for the severe amplitude distortion that is experienced by the higher speed modems. The problem with the implementation of decision feedback equalization is that it does not operate harmoniously in systems using error correction coding as will now be explained.
Error correction coding is a coding technique used to increase the immunity of a digital information signal to the presence of noise. Such increased immunity, in turn, increases the probability of accurately recovering the information signal in the receiver unit of a data communications system. Error correction is typically characterized as being either block or convolutional coding. Trellis coding is one well-known error correction coding technique that utilizes convolutional coding and does not affect the bandwidth required of the communications system.
In block coding, one or more error correction bits are transmitted along with a "block" of one or more information bits. Each of these error correction bits has a value which is determined by the value of the information bit or bits in the associated block. This process of transmitting an expanded number of bits for error correction is also utilized in convolutional coding but, unlike block coding, the value of each bit in convolutional coding is a function of the information bits in the associated block and a number of priorly transmitted blocks.
Coding gain is a term which refers to the increased performance of a system resulting from the use of error correction. It is defined as the amount by which the signal-to-noise ratio may deteriorate before the bit error rate equals that of the same system without error correction. This term can be calculated analytically for any system and, for purposes herein, the resulting quantity is referred to as the theoretical coding gain.
As decision feedback equalization and error correction coding each address different undeirable effects in digital communications systems, namely amplitude distortion and noise, respectively, the combination of both techniques should provide a greater benefit than either technique alone. It has been found, however, that when decision feedback equalizers are operative upon digital signals incorporating error correction, system performance is degraded. Indeed, the resulting performance can be substantially below that obtainable with the use of either error correction or decision feedback equalization alone. Accordingly, a significant communications improvement would result if the combined benefits of both decision feedback coding and error correction coding could be obtained in a single communications system.